Magnesium base alloys



Patented Dec. 3, 1940 UNITED STATES PATENT OFFICE MAGNESIUM BASE ALLOYS No Drawingpplication March 16, 1940, Serial No. 324,402. In Germany March 13, 1939 '7 Claims.

This invention relates to magnesium base alloys and is especially concerned with raising the yield point of certain cast alloys.

For all shaped casting which are required to show accuracy to gage under mechanical stress, the yield point of the material is, next to the endurance properties, the propertyupon which the degree of permissible working stress principally depends. In particular when light metals are employed for the production of motor parts, for aircraft construction and other vehicle drivers, plastic changes of shape are not admissible in operation even to the least degree. Raising the yield point of the material employed therefore allows of higher permissible stresses and tlrus offers the advantage of a lower constructional weight.

The technical problem of improving as far as possible the mechanicalproperties of light, metal shaped castings has found a useful solution in the case of numerous alloys by their capacity for improvement by heat treatment. Whilst, however, these thermal refining processes have on the whole shown satisfactory results in the field of aluminium alloys, the thermal improvement treatment of shaped magnesium castings has not hitherto fulfilled expectations.

At present there is in practice especially one thermal refining process in use for magnesium alloys,. according to which a heat treatment (called a solution heat treatment or homogenisation. heat treatment) is carried out at a temperature of 400 C. or a little thereabove asfa first stage of treatment, with subsequent quenching or chilling. An annealing treatment at a lower temperature, preferably below 200, 0., may;

follow as a second stage. 7 This two-stage process offers a series of dangers to the shaped casting. At the high temperature of the solution heat treatment the piece under treatment has practically no rigidity left and tends to distort unless it is very carefully suppored. The danger of fire is great at this temmainder of the casting. The range of distribution of strength values is accordingly unsatisfactorily high. The yield point is practically unaltered by the solution heat treatment' and quenching. It can only be raised by a subse- 5 quent annealing.

It is an object of the invention to provide a process. for raising the yield point of high per cent. magnesium alloys, which contain magnesium as their greatly predominating component and aluminium and manganese as additional metals and may further if desired contain zinc also, by simple annealing (artificially ageing) at temperatures between and 300 C. Another object is to avoid the necessity for using the two-step thermal treatment which has theretofore been the standard practice in treating castings of this type.

Heat treatment at such low temperatures leaves the strength of the material under treatment as. good as unaltered, the elongation even decreases slighly; on the other hand the yield point is very considerably raised by it. Thus, compared with the above-described two-stage process, the thermal refining of magnesium alloys according tothe invention deliberately abandons the increase in strength, which cannot indeed be obtained uniformly by the solution heat treatment with subsequent quenching and moreover is associated with the danger of rejects reaching a very high proportion, and instead of this by a simple, safe and economical process raises the yield point, as being the most important property for the permissible stress. This eifect of the single step heat treatmentmay be achieved with all usual manganese-mag-nesium alloys which besides a con tent of 0.1 to 2% of manganese contain from 1 to at most.7% of aluminium, orfa mixture of from 1 to 7% of aluminium with less than 4% of zinc. Small quantities of other elements can be added to the alloy, provided the fundamental character of the alloys and'the influence of additions of manganese and aluminium (and if desired also of'zinc) withinthe limits given is not altered. Thus a small proportion, preferably from 0.5 to 2%, of antimony or bismuth or of both metals may serve to improve the corrosion properties.

It is the'preferred practice to perform the heat treatment at temperatures from to 250 degrees C. At these temperatures the castings should be treated for a period of about 24 to 72 hours or more. The alloys may be made in the usual manner and then melted and cast under the conditions usually prevailing in magnesium alloy foundries.

Owing to the low temperatures of the heat treatment, precautions are not needed to ward off the danger of the casting taking fire. Just so no special provision for supporting the casting is required: the castings are simply put into a furnace chamber maintained at the requisite temperature, and then taken out therefrom after the expiration of the predetermined period of treatment. Seeing that this improvement treatment consists of one operation only and does not involve the danger of refuse, the cost f of this thermal refining method is very low. At the same time the casting is relieved of any existing casting strains.

It is already known (Gann and co-applicants, U. S. A. Patent No. 1,936,550) that high per cent. magnesium alloys, which contain more than 0.5%, preferably more than 1%, of manganese, are improved remarkably as regards their yield point and not inconsiderably as regards their hardness by a heat treatment at temperatures between 250 and 500 0., preferably between 300 and 400 C., for aperiod of 1 to 40 hours, preferably 1 to 24 hours. The same applies to high per cent. magnesium alloys which in addition to more than 0.5% of manganese (which amount corresponds to the solution equilibrium at normal temperature) also contain another, additional metal or several additional metals. It-is assumed that the manganese is precipitated in elementary form by this heat treatment (see page 2, lines 82 to 85.) Special conditions arise when the further additional metal goes into solid solution at the temperature of treatment (see page 3, lines 36 to 113), as is the case with zinc which forms an eutectic with magnesium having a melting point of 344 C. A content of 3% zincalone 40 does not exert a hardening effect on heating the binary zinc-magnesium alloy to 320 C. and only produces a very small rise in yield point. However, in the presence of 1.3% of manganese the Brinell hardness is raised from 37 to 42 and the 45 0.1-yield point from 3.85 kg./mm. to 5.46

kg./mm. (see the ,table. on page 3, lines 50 to 60 of the UrS. A. patent), and this eifect can be further considerably improved if the whole excess of zinc over the quantity corresponding to solu- 50 tion equilibrium or a part thereof is again precipitated by a subsequent heating to a lower temperature (e. g. 150 to 200 C.) (see page 3, lines This known process depends upon the assump- 5 tion that it requires a heating to 250 to 500 C.-

preferably 350 C.in order to bring about the rise in yield point by precipitation of manganese, whether or not other metals are present in the alloy, and that even these relatively high tem- 60 peratures only produce the eiiect when more than 0.5% of manganese, preferably more than 1.0% of manganese, is present in the alloy. In

the case of zinc, zinc is simultaneously brought into solid solution at the temperature which pro- 65 duces precipitation of manganese, a part of which zinc is preferably to be precipitated again by a second heat treatment at lower temperature.

In the present process the precipitation of manganese, so far as the eifect would be ob- 70 tained by such a precipitation, is apparently promoted by the presence of considerable quantities of aluminium (or aluminium and zinc) in such a way that it occurs at temperatures below 250 0., preferably at 175 C. or even therebelow,

75 and this even when the manganese content does not reach 0.5% so that the unfavourable effects of a heat treatment at higher temperatures are avoided. At this lower temperature on the other hand changes in state of the aluminium (or of the aluminium and zinc) simultaneously take place, which lead to a reduction of the proportions present in solid solution without an additional heat treatment being necessary for this. Moreover the yield point of the alloys thus made up is considerably higher even in the as cast condition of the alloy, so that through the further rise which is attainable by the heat treatment according to the invention, final values are obtained which very considerably surpass the results of the known process.

On the other hand, it was already known from U. S. A. Patent No. 1,592,302 that magnesium alloys which contain aluminium up to 12% (aluminium is soluble to the extent of 11 to 12% in solid magnesium at the eutectic temperature of 435 C.) can be homogenised by solution heat treatment at temperatures above 400 C., whereby the tensile strength and plasticity of the alloys is raised whilst on the other hand the Brinell hardness and also the elastic limit and the yield point fall (see page 3, lines 1 to 5). The homogeneous state of the solid solution is said to be preserved to a certain extent by subsequent quenching, whereupon a considerable hardening of the alloy can finally be produced by a further heat treatment at lower temperatures (125 to 300 C.) in which a part of the aluminium is precipitated from the solid solution in the form of very fine crystals. (There is no mention of a favourable effect by this annealing on the other properties, especially the yield point.) This known process also takes place in two stages and is therefore associated with the disadvantages originally mentioned. Further, the prior specification admits that the hardening obtained is insignificant with an aluminium content of 6% or less, and that an improvement only begins with an aluminium content of 8%, reaching the highest value at 12% (see page 1, lines 39 to 49) With an aluminium content of less than 4% hardening was only observed when 4% of zinc was also at the same time present in the alloy. The interior solution heat treatment at temperature above 400 C. is indeed not set out in the specification as a necessary prerequisite for success; smaller, but still noticeable effects as regards increase in hardness are said also to be obtainable when the casting immediatelyafter its natural cooling is exposed to a heating of appropriate duration at 125 to 300 C. It is, however, added (see page 3, lines 53 to 62) that this is only the case when a suflicient quantity of aluminium remains behind in solid solution as a result of special conditions of cooling. Of six examples, only two (Examples 4 and 6) relate to such a one-stage treatment whilst Example 4 treats extruded alloys, so that actually only Example 6 refers to the onestage improvement treatment of cast alloys. This example relates, however, to the thermal refining of a casting of a magnesium alloy with 12%. of aluminium. It is also admitted that even under these conditions the rate of increase in hardness was much slower than in the case of using the two-stage process with heating to 420 C., with subsequent water-quenching, and that the final hardness reached was also less than the value obtained by the two-stage treatment (see page 2, lines 68 to 72) On the other hand the rise in yield point (without indeed appreciable hardening of the shaped casting) which is the exclusive object of the present process, is obtainable with a proportion of aluminium of up to 7 which may fall to 2% (and even still lower). ment between 100 and 300 C. under these circumstances, however, only leads to its object when besides aluminium (or aluminium and zinc) manganese is also contained in the alloy. Whether or not a material contribution is made to the rise in yield point by precipitation of manganese, the presence of this further additional metal in quantities of 0.1 to 2% does cause that improvement of the alloy, which is the object of the invention, to occur to a very considerable degree even with so low an aluminium content.

Examples 1. Test bars with a diameter of 12 mms. of an alloy containing 6% of aluminium, 3% of zinc and 0.3% of manganese, the remainder being magnesium, were cast in sand and annealed for two days at 175 C. As a result of this treatment the 0.1%-yield point rose from 8.65 to 11.2

, kg./mm. the 0.2-yield point from 10.25111) 12.8

kg./mm.

2. Test bars of 17 mm. diameter of an alloy containing 4% of aluminium, 3% of zinc and 0.3% of manganese, the remainder being magnesium showed in the as cast condition a 0.1- yield point of 6.1 kg/mm. and a 0.2-yie1d point of 7.55 kg./mm. After an annealing treatment at 175 C. for 24 hours the corresponding values were 7.8 and 9.45 leg/mm.

3. The mechanical properties of two test bars cast together from an alloy containing 3.5% of aluminium, 2% of zinc, 0.2% of manganese and 0.3% of antimony, the remainder being magnesium, were determined, one in the cast condition, the other after a two-day heat treatment at C. The 0.2-yield point amounted to 7.95 kg./mm. in.the cast condition, as against 9.5 kg./mm. for the annealed bar. The strength and extension were determined as 19.8 kg./mm. and 8% for the cast bar and 19.9 lag/mm and 6.5% for the annealed bar.

4. A cast alloy containing 7% of aluminium, 0.3% of manganese and 0.3% of antimony, the remainder being magnesium, showed an increase in the 0.1-yield point of from 8.4 to 9.8 kg./mm. and in the 0.2-yield point of from 9.9 to 11.8 lIg/mm. on annealing for three days at C.

5. With a cast alloy containing 2% of aluminium, 0.3% of manganese and 0.3% of antimony, the remainder being magnesium, the same treatment gave an increase in the 0.1-yield point of from 4.2 to 5.9 kg./mm. and in the 0.2-yield point of from 5.25 to 6.9 kg./mm.

6. With a cast alloy containing 7% of aluminium and 0.7% of manganese, the remainder being magnesium, an increase in the 0.1 yield point of from 8.25 to 10.2 ken/mm? was .obtained by annealing at 240 C. for 23 hours, whilst the 0.2-yield point was raised from 9.7 to- 12.1

The single heat treat- What we claim is:

1. The process of improving the yield point of cast magnesium base alloys containing from 1 to 7% aluminium and also containing from 0.1 to 2% of manganese, which process comprises subjecting the casting in the as cast condition to a single step heat treatment consisting in simple annealing at a temperature between 140 and 250 degrees C. for about 24 to 72 hours or more.

2. The process of improving the yield point without appreciable increase in strength of cast magnesium base alloys containing from 1 to 7% aluminium and also containing from 0.1 to 2% of manganese, which process comprises subjecting the casting in the as cast condition to a single step heat treatment consisting in simple annealing at a temperature between 100 and 300 degrees C.

3. The process of improving the yield point without appreciable increase in strength of cast magnesium base alloys containing from 1 to 7% aluminium and less than 4% zinc, and also containing from 0.1 to 2% of manganese, which process comprises subjecting the casting in the as cast condition to a single step heat treatment consisting in simple annealing at elevated temperatures between 100 and 300 degrees C.

4. The process of improving the yield point without appreciable increase in strength of cast magnesium base alloys containing from 1 to 7% aluminium, less than 4% zinc, and from 0.05 to 2% ofat least one additional metal from the group consisting of antimony and bismuth, and also containing from 0.1 to 2% of manganese, which process comprises subjecting the casting in the as cast condition to a single step heat treatment consisting insimple annealing at elevated temperatures between 100 and 300 degrees C.

5. An alloy containing magnesium as the principal metal and containing from 1 to 7% aluminium and also containing from 0.1 to 2% of manganese, and characterised by the yield point of the shaped casting having been raised without appreciable increase in strength by simple annealing at a temperaturebetween 100 and 300 degrees C.

6. An alloy containing magnesium as the principal metal and containing from 1 to 7% alu- -minium and less than 4% zinc, and also containing from 0.1 to 2% of manganese, and characterised'by the yield point of the shaped casting having been raised without appreciable increase in strengthby simple annealing at elevated temperatures between 100 and 300 degrees C.

7. An alloy containing magnesium as the principal metal and containing from 1 to 7% aluminium, and less than 4% zinc, and from 0.05 to 2% of at least one additional metal from the group consisting of antimony and bismuth, and also containing from 0.1 to 2% of manganese, and characterised by the yield point of the shaped casting having been raised without appreciable increase in strength by simple annealing at elevated. temperatures between 100 and .300 degrees C.

FRITZ JABUREK. GEORG SCHICHTEL.

CERTIFICATE OF CORRECTION. Patent No. 2,225,852. December 5, 19m.

- FRITZ JABUBEK, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line LL, for the word "casting" read -castings-; and second col-F umn, line 14.6, for "O. 5" read --0.05--; page 5, first column, line 67 Example 7, for "O. 5" read O.'5C'Z;- and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this lLLth day of January, A. D. 19in.

. Henry Van Arsdale, (Seal) Acting Commissioner of Patents. 

